1. Field of the Invention
The present invention relates to an inorganic nonaqueous electrolytic solution type cell comprising an oxyhalide which serves as an active material for positive electrode and also as a solvent for an electrolytic solution.
2. Description of the Related Art
An inorganic nonaqueous electrolytic solution type cell, which comprises an oxyhalide (e.g. thionyl chloride, sulfuryl chloride and phosphoryl chloride) as an active material for positive electrode, an alkali metal as a negative electrode, a porous carbon molded article as a positive electrode and in which the oxyhalide serves as a solvent for an electrolytic solution, has a large energy density and works at a lower temperature. However, since the oxyhalide has strong oxidation activity, a separator should be resistant to the strong oxidation activity of the oxyhalide.
For this reason, in the inorganic nonaqueous electrolytic solution type cell, vinylon-rayon mixed paper or vinylon paper which is used in an alkali cell cannot be used as a separator, and a nonwoven fabric of glass fibers having good oxidation resistance is used (cf. Japanese Patent Kokai Publication No. 121563/1983).
Since the nonwoven glass fiber fabric is not oxidized with the oxyhalide, it is stable in the electrolytic solution and has a long life. However, since the glass fibers have no inherent adhesivity and are bound through entanglement among them, the nonwoven glass fiber fabric has small mechanical strength such as tensile strength. Therefore, the separator made of nonwoven fabric tends to be broken during assembling of the cell and induces internal short circuits.
To increase the mechanical strength of the nonwoven glass fiber fabric, an organic binder resin such as polyethyl acrylate is mixed with the glass fibers when the nonwoven fabric is produced.
When the cell comprising the separator made of the nonwoven glass fiber fabric is stored for a long time or at a high temperature, the organic binder in the nonwoven fabric is gradually dissolved in the electrolytic solution so that the separator loses its strength and cannot maintain its shape. As the result, the separator has irregular thickness and thinner parts of the separator have decreased ability for retaining the electrolytic solution. Then, it becomes difficult for the alkali metal to migrate from the negative electrode to the positive electrode through the thinner parts of the separator, whereby a closing voltage of the cell decreases and in some cases, the short circuits are formed at the thinner parts of the separator.
To avoid the above problems, it may be contemplated to use a porous resin separator having good oxidation resistance and electrolytic solution-retaining ability. Examples of such porous resin film are a microporous film of ethylene-tetrafluoroethylene (ETFE) copolymer or polytetrafluoroethylene (PTFE).
When the PTFE microporous film as such is used as the separator in the cell, fluorine atoms in the polytetrafluoroethylene film react with the alkali metal of the negative electrode such as lithium and the film loses functions as the separator.
Since the pores in the ETFE or PTFE microporous film are uniform micropores and have complicatedly curved passages. Therefore, such pores are favorable for preventing migration of carbon particles but has smaller electrolytic solution-retaining ability than the nonwoven glass fiber fabric since the pores in the ETFE or PTFE porous film are not three-dimensional pores. Therefore, the amount of the electrolytic solution retained in the separator is insufficient so that sufficient discharge performance of the cell is not achieved.